Electro-optical device



Jul 1, 1947. G. K. TEAL ELEGTRO-OPTICAL DEVICE Filed Jan. 30, 1945 SECONDARY [memo/vs INVENTDR By G. K. TEAL A ORA/E V Patented July 1, 1947 ELECTED-OPTICAL DEVICE Gordon K. Teal, Summit, N. J., assignor to Bell Telephone Laboratories, Incorporated, New York, N. Y., a corporation of New York Application January 30, 1943, Serial No. 474,205

8 Claims.

This invention relates to electro-optical devices and, more specifically, to photo-E. M. F. devices for controlling electron beams.

A photo-E. M. F. device may be defined as a material, combination of materials, or cell which when electromagnetic radiations of certain short wave-length, like those, for example, to which the eye is sensitive, are applied thereto will produce an electromotive force across its terminals, or across two portions thereof which can serve as terminals. In the present invention, in one of its primary aspects, the voltage set up between the terminals of a photo-E. M. F. device or cell when such waves are applied thereto is used to control an electron beam. In describing the invention, such waves will be called light, but this term is used in a broad sense to be inclusive of radiations to which the eye is not sensitive. It is an object of the present invention to provide a novel arrangement wherein electrons of a beam or stream are controlled by the voltage appearing across the terminals of a photo-E. M. F. device in response to light radiations applied thereto.

It is another object to provide a novel electrooptical device in which there is included photo- E. M. F. means for controlling secondary electrons.

In accordance with the invention, there is provided an electro-optical device wherein a beam or stream of electrons is controlled by means of potentials generated by photo-E. M. F. elements, which photo-E. M. F. elements are subjected to applied light radiations.

In accordance with a preferred embodiment, chosen by way of example to illustrate the principles of novelty of the present invention, a television electron camera tube is provided which comprises an evacuated envelope enclosing means for generating a beam of electrons, and an apertured target for said beam comprising an apertured metal screen or plate carrying on the side remote from the beam generating means a multiplicity of elemental photo-E. M. F. cells. An image of an object is projected upon the photo- E. M. F. cells and the other side of the target is scanned with the beam of electrons.

The principle of operation of this tube is as follows: The beam generating means produces a relatively high velocity beam and this is caused to scan the side of the target remote from the photo-E. M. F. cells. These cells have applied thereto light radiations from the object to be televised and there is produced across each cell a voltage proportional to the light striking the cell from a corresponding elemental area of the object. The scanning beam strikes the target plate and causes the emission of secondary electrons which pass through the apertures in the target member to a collecting electrode placed within the tube between the target and the object. The number of electrons which pass through any aperture is controlled by the voltage across the neighboring elemental photo-E. M. F. cells. The collecting electrode is connected to a signal resistor through which passes the video current, which is amplified by any suitable amplifier.

While the invention in its principal aspects relates to electron camera tubes for television or to novel targets therefor, it will be appreciated that it is not limited to this, as the principle of controlling a beam of electrons (and more specifically the fiow of electrons through an apertured memher) in response to the voltage produced by a photo-E. M. F. cell can have other uses, as in light-controlled devices for switching, for example.

The invention will be more readily understood by referring to the following description taken in connection with the accompanying drawing forming a part thereof, in which:

Fig. 1 is a schematic representation of a cath-* ode ray tube of this invention and certain of its associated circuits;

Fig. 2 is a schematic view showing, in greatly enlarged form, a. portion of the target and of the electron collecting electrode of the tube shown in Fig. 1; and

Fig. 3 is an enlarged front view of a portion of the target.

Referring more particularly to the drawing, Fig. 1 shows, by way of example to illustrate the invention, a cathode ray television transmitter tube Iii employing a two-sided mosaic target I! containing photo-E. M. F. elements. The tube ii! comprises an evacuated container l2 enclosing the mosaic target I i, an electron gun l3 for generating, f ocussing and accelerating a beam of high velocity electrons towards this target, a secondary electron collecting electrode M on the side of the target ll remote from the electron gun l3, and two sets of electrostatic deflecting plates l5 and [6 for causing the beam of electrons to scan every elemental area in turn of a, field of view on the mosaic target ll. Radiations from an object or field of view 0 are applied to the side of the mosaic target I l remote from the electron gun by means of any suitable optical system represented schematically by the lens I1.

The electron gun I 3 preferably comprises a material or, if desired, it may consist oi a ring of metallic material.

The control electrode 2| is placed atany suit-; able negative potential with respect to the potential of the cathode by means or" an adjustable source and the first anode 22 and the final anode 23, 2d are placed at appropriate positive potentials with respect to the cathode 20 by means of the source 3| and the source 32. As an example the final anode 23, 24 can be about 1,0Q0 volts positive with respect to the cathode and the first anode 22 can be 300 volts positive with respectto the cathode. Any suitable source can be utilized to heat the cathode 29. The negative terminal of the source 3| is connected to the cathode 211 and the positive terminal thereof is; connected to the first anode 22', while the negative terminal or the source 32 is connected to the positiveterminal of the source 3! and the positiveterminal of source 32 is connected to the second anode 23; 24; Preferably the positive terminal of the source is connected to ground through a source 31 which is used to make the apertured plate (see Fig. 2) of the target positive withr'spect to thefinal anode of the electron gun for a purpose which will be pointed out hereinafter. The voltage of this source 31 can be, for example, 20130 50 volts. The aperturd plate; 5|lis connected to ground and is also connectedtl'lrough asource 33 and a signal resistor 34 to the collecting electrode l4. Any suitable amplifier f5 is connected to the signal resister stand i s-i connected to the other elemerits or the television transmitter circuit which prepare the video currer'lt for transmission to the receiving station. The potehtials app-lied to the various electrode members and their configuration and shape are such that a beam of fo'cussed high velocity electrons strikes the target II and this beam is deflected over a suitable field thereon by m eans of ppropriate potentials applied to the deflecting plates F5 and i6 by electrostatic sweep circuits (not shown) to produce secoildary electrons, the action of which will be considered more fully below, As examples of satisfactory sweep circuits, reference may be made toPaterlt No. 2,178.4}34, dated October 31, 1939, to M. w. Baldwin, Jr., which discloses balanced electrostatic sweep circuits suitable ror this purpose. Connections can bemade from the balanced sweep circuitsto the pairs off plates I5 and -l6 by means of coupling condensers in, 4] and 42, 43, respectively of about one microf'arad capacity each. Coupling restitances M and 45 of the order of many megohms each are respectively connected across the pairs of p1a't e's l5 and IE. The midpoints of the resistances 4'4 and 45 are connected to the positive terminal of the source 32 so that the average of the potentials of the deflecting plates does not deviate more than slightly from the potential of the anode 23, 24. This relationship is maintained tcravcia ch" flees in the sensi tivity of the deflecting system and the consequent distortion of the image which would otherwise result. For more complete descriptions of the rated screen or plate 55] an el 4 advantages of balanced sweep circuits for use with cathode ray television tubes, reference is made to the above-mentioned Baldwin patent and also to Patent 2,209,199, issued July 23, 1940, to Frank Gray.

Reference will now be made to Figs. 2 and 3 which are enlarged schema-tic views of a pref'errd form of mosaic target ll. Fig. 2 is a schematic side view, in cross section, of a portion of the target I I, while Fig. 3 is an enlarged showing of a portion of the target I I, viewed from the right inFig; 2, to show the general relationship of the apertures '52 in the target and the photo- E; M F. cells 5| thereof, Certain dimensions in Figs. 2 and 3 have been exaggerated at the expense of others in order to more clearly show the screen structure. The mosaic target preferably comprises an apertured plate 50 of any suitable material, such as nickel, carrying thereon on the side remote from the beam generating means arriultiplicity of photo-E. M. F. cell elements 5|, each ape'rtiire 52 in the plate being eiiectively surrounded by cell elements. Each of the eleine'rits 5! can comprise, for example, a small copper oiilde photo-E. M. F. cell which cell, as is well known in the art, generall comprises a layer of copper, a semiconducting layer of oilprous oxide thereon treated by means well known in the art to produce on the semiconducting laye a blocking layer, and a thin semitransparent conducting element such as silver or gold covering the blocking la er.

A satisfactory method or making the target structure U of the tube is as followsi A perfoor nickel or any other suitable metal, made by any well-known commercial process, is insufilated with a suitable protectiv'e' covering material, suc as asphaltum particles small in comparis n with the diameter of the screen. The screen 50 has, for examp1e to 200 or more apertures per inch. It is slightly warmed to cause the asphaltlim deposit to new and produce a multiplicity of dot-like elements fairly evenly distributed over the surface of the plate. The back or theipl'ate is tnen ccvel-ed with asphaltum which can be done without the asphaltum going completely through to the insur flated side by pouring asphaltum varnish of the correct consistency on the back of the screen and allowing it to dry. A silver or other Suitable r'riet'allic mask is then plated on the unprotected areas of the plate 5'0, a thin greasy or waxy coatin'g having been first applied to theta'rg'et so that the silver can be later stripped clr. All the as' phaltum is then dissolved in benzene. A copper fil'r'n is then deposited on the plated side of the member copy evaporation, "The silver mask or screen is then carefully stripped away leaving small dots of copper where the asphaltuin dots had fornierly been. The copper dots are then oxidized by any suitable means and the oxide surfaces treated by electron or ion bombardment to produce a blocking layer for each small cell. A very thin semitr'anspar'ent of gold or other suitable conducting materialis evaporated onto the small cell surfaces. By this process there is produced a number of small sandwich-type of photo-E. M. F. cells distributed at random over thescreen. If desired, the copper dots can be oxidized, treated, and the semitrans'parent film evaporated thereon before the silver mask is stripped off. v These cells are or the type known as a ffron't wall cell. In these cells a negative p6 tjential with respect t'o the plate 59 is acquired by the semitransparent metal layer when light radiations are applied to the cells from the object 0. If desired, the cells can be produced by any suitable process which produces a back wall type of cell but cells of this latter type are generally not as sensitive as those'of the front Wall type. If the cells are of the back wall type, the semitransparent metal layer becomes positive with respect tothe plate v50 when light radiations are applied to the cells. The arrangement in accordance withthis invention is operable with either typeof 'cell and, moreover, is not limited to the use of the copper oxide type or of the silicon-silicon oxide type now to be described.

If desired, the surface of the target nearer the beam can be treated in a manner to enhance the production of low velocity secondary electrons by evaporating a thin layer of a suitable metal, such as magnesium, on this surface and then wholly or partially oxidizing this metal layer.

As an alternative, the cells can be of the siliconsilicon oxide type. These cells can be made as follows: First, silicon (instead of copper) is deposited on the silver-plated side of the mosaic after the asphaltumhas been dissolved. A preferred method is to deposit the silicon in two layers, one by simultaneous evaporation of silicon and amore conducting element, such as aluminum, arsenic, titanium, copper or silver, and the second by evaporation of silicon alone. The effect of the more conducting element is to give the cell a lower internal resistance than would otherwise be obtained. The silicon (or silicon and the more conducting element) layer is then covered by evaporation of quartz (silicon dioxide) or anothersuitable insulator with tungsten or another suitable element such as silicon, molybdenum, or tantalum. The evaporations can be efiected either simultaneously or in alternate thin layers as the layers are very thin and not very smooth, thus permitting one to partially merge with the other. Finally, a thin semitransparent film of gold, silver or other suitable conducting material is evaporated onto the small cell surfaces and the silver mask and the films on it are stripped away, leaving small sandwich-type silicon-silicon oxide photo-E. M. F. cells distributed at random over the screen. The silicon-silicon oxide cell described above is of particular advantage in the camera tube in accordance with thi invention because of its low impedance. Because of this, electrons which fall back on the surface of the target remote from the beam do not charge this surface.

The operation of the arrangement shown in Fig. 1 is as follows, reference also being made to Figs. 2 and 3. Radiations from an object or field of view are projected upon the right-hand side of the mosaic target I i by means of the lens system H. A high velocity beam of electrons generated by the electron gun I3 is deflected over a field of the mosaic target ll corresponding to the area covered by the radiations from the object by means of deflecting potentials applied to the deflectin plates and I6. As shown in Fig. 2, the cathode ray beam is generally of such size that it covers several apertures 52 and the spaces between them. It will be appreciated that the lines indicatin the beams are not accurately representative of the electron paths since obviously the electric fields adjacent the apertures of the member 50 are not uniform and produce a certain amount of convergence or divergence. When the primary beam strikes the metal member 50, secondary electrons are given off and these secondary electrons (mixed .with'some primary electrons which are not shown in the drawing in orderto simplify'it) pass through the aperture or apertures 52 adjacent the scanning spot to the collecting electrode M. The source 37 places the plate 50 at a positive potential of from 20 to 50 volts with respect to the electrode 24 to allow only electrons originating next to an aperture to go through it, or, in other words, a field is established which discourages secondary electrons from traveling from one point of the target to another. The number of secondary electrons which pass through any particular aperture 52 is dependent upon the potential generated across the adjacent individual photo-E. F. cells 5| by the radiations applied thereto from the corresponding elemental area of the'object. The action of the photo-E. M. F. cells 5| is somewhat analogous to that of a grid in a triode. This voltage between the semitransparent metal member and the metal screen member 50 varies the number of secondary electrons which pass through the corresponding aperture 52. .This voltage has practically no effect on the primary electrons but as the number ofxthem passing through the target II in a given time interval remains substantially constant, these electrons contribute only to the .average or background current flowing through the resistor 34 and do not affect the signal current. Moreover, secondary electrons emitted from the plate 50 which do not go through the apertures 52 and are picked up by the plate 50 do not introduce distortion in the signal current as the plate 50 is maintained at a fixed potential. If any secondary electrons fall back on the Side of the cells 5| remote from the electron beam, they are immediately conducted to the plate 50 as the resistance of the cells is relatively low, Unlike the storage type of camera tube using photoemissive elements wherein ,each photoemissive element mustbe discharged once per cycle,'the photo-E..M.v F. .cells 5! can have the potential thereacross varied at will (by the change in light) and need not have this potential brought to zero once every scanning cycle. The variable current passing through the resistor 34 constitutes the video signal current and this current is amplified by the amplifier 36 in a manner well known in the art. The video current is then caused to modulate a carrier for transmission to a distant station by means well known.

Various modifications can be made in the embodiment described above without departing from the spirit of the invention, the scope of which is indicated by the appended claims.

What is claimed is:

1. An electron camera tube target comprising a metal base member, and a multiplicity of relatively small photo-E. M. F. cells on said member, each of said cells comprising silicon and an oxide of silicon.

2. An electron camera tube target comprising a metal base member, and a multiplicity of relatively small photo-E. M. F. cells on said member, each of said cells including a layer comprising silicon and a more conducting element than silicon and a layer of an oxide of silicon.

3. The combination of an envelope containing an electrically conducting plate-like member having a multiplicity of minute apertures therethrough with small photo-E. M. F. devices adjacent thereto and for which said member serves as a common terminal, the second terminals of said devices :being an arrayron oneside of saidmember and .no external electrical connections thenetogsaid array being exposable to light,therie being :at :least one of said devices adjacent each of said apertures, means within said envelope :for producing :a flow of electrons through said apertunes and past said array, means :separated from said member 'and positioned on the same side thereof as the array for collecting electrons 'ai-ter they'pass said array,vandvmeans c'onnected tozsaid member i or "maintaining the potential'thereof independent -of "the potential or potentials of :the second terminals -"of said devices.

:4. An electron space current device for converting light variations into electrical variations comprising an enclosing envelope rhavin'g therein a mosaic electron target comprising an aperture'd conductive member carrying iamultiplicityof discrete photo-E. F. elements in conductive relation thereto and adjacent 'the apertures thereof, a wall of said'envelope being light conducting to permit light to be impressed on said cells from Without the envelope toenergize the cells to set up voltages therea'cross dependent upon theintensity of the :light incident thereon, :and means aligned with said target for generating -electrons of such low velocity that their flow can be controlled by voltages of the order 'of.magnitude of these produced -:by said cells, and "means adjacent said target for setting up a :Eorcewithin the space adjacent said apertures for directing some of said low velocity electron-s toward each .of :said .apertures :in

5. The combination of elements as in claim i in which said photo-E. v1 elements are contignous to one side :of :said target (and-the means for generating the low "velocity electrons comprises means for generating a beam :of high velocity electrons and for directing it so that it impinges upon sthesside of said target remote from the photo-E. M. F. elements, thereby causing emission from saidiconducting member-f secondary electrons which pass through said apertures 8 under control :of fbhe voltages set up across irhe photo-E, :elements adjacent the corresponding :apertures.

=6; The combination of elements as in claim 4 in which each'o'f said discrete photo-E. M.-:el.emerits includes "silicon.

"7. The combination of elements as in claim .4 in which each of :said "photo-E. :elements comprises :silicon -and an oxide of silicon.

3. The rcombination iOf elements as in 'claim *4 in which each of said discrete photo-1E. M. :F. 4518- ments includes a layer comprising silicon :and :an element more conducting than'siliconeanda layer of an :oxide of silicon.

GORDON The following references are of record in the file of this patent:

UNITED :STATES PATENPI S Number Name Date 25045984 Flory I-une 3031 936 231161901 Knoll May 1 0, 1 938 21755691 Kiam-s @012. 10, 1 939 2,2 12; 92'3 Miller Aug. "27, 1 940 223 623808 Gall-up "June ZO, '1-9'39 2,1-40394 Gohl'ih Dec. 20, {E938 2,175,791 "Rose Oct. "1 0, 1939 '2,' 1' 7 5;689 Gallup Oct. 10, 19-39 2,141,375 'Ca-stro Dec. 2 7, 1938 2304;25-1 Genes June -1'=l,'1"940 1907124 Ruben May 2, 1 938 2, 1201765 Orv in June 14, 1938 1935514 Lnegnik Nov. 21, T933 1343395 Pfiffer Jan. 1%, 19'34 2,324,505 'Iams el; al. July 20, 1"943 2530557 6 'Lan-ib Dec. 15, 1942 250951782 Wilson Oct. 12, 1 937 I FOREIGN PATENTS Number Country Date 7835646 France 8, 15135 

